1. Field of the Invention
The present invention relates to a method for fabricating a thin-film transistor and, more particularly, to a method for fabricating a reverse-staggered thin-film transistor employed in a liquid crystal display device.
2. Description of the Prior Art
FIG. 1 shows a conventional thin-film transistor of an amorphous silicon. This thin-film transistor is constituted by a gate electrode 2, a gate insulating film 3 made of a silicon nitride film, an intrinsic amorphous silicon layer 4 and an n-type amorphous silicon contact layer 5, a drain electrode 6, a source electrode 7. A protective layer 9 made of silicon nitride is further formed to cover the transistor. In this structure, the surface portion 10 of the amorphous silicon layer 4 between the drain electrode 6 and the source electrode 7 is sometimes called a "back channel" and has one of key factors determining the characteristics of the transistor.
Specifically, if back channel 10 is contaminated and thus charged prior to the formation of the protective film 9, a leak current is caused to increase and flows between the source and drain. Even if the back channel 10 is not contaminated, the positive electric charges due to the contamination on the protective film 9 increase an electric field applied the back channel 10 up to or more than the threshold of the back channel. A leakage current thereby flows. In addition, the protective film 9 itself may be positively charged, so that the leak current also flows.
In order to overcome such a problem, a thin-film transistor is disclosed in Japanese Patent Laid-Open No. Hei 2-163972 that has a back channel covered with an oxide film obtained from hydrogen peroxide or with an alumina film formed by the CVD process. Such a film can convert the back channel into a p-type one. The leakage current is thereby decreased.
However, the p-type back channel thus converted depends on the quality of the oxide or the alumina film and is thus inferior in controllability. Moreover, when hydrogen peroxide is used, the drain and source electrodes are damaged by the hydrogen peroxide. In the case of the alumina film, on the other hand, alumina in the alumina film may be diffused into the amorphous silicon layer to change the property of a thin-film transistor.
Another thin-film transistor is disclosed in Japanese Patent Laid-Open No. Hei 4-321275 to reduce the leakage current. In this transistor, an aluminum oxide layer is formed on a back channel. For this reason, this transistor also has the same problems as mentioned above.
Turning to FIG. 2, there is shown another thin-film transistor which is disclosed in Japanese Patent Publication No. Hei 5-0083197, in which the same constituents as those shown in FIG. 4 are denoted by the same reference numerals. In this device, a portion of the n-type contact layer 5 which is not covered with the drain and source electrodes 6 and 7 is converted into a p-type region 11 by adding p-type impurities therein. The back channel is thereby covered with the p-type region 11.
However, the contact layer 5 has a considerably high impurity concentration of 10.sup.20 atoms/cm.sup.3 or more. For this reason, it is necessary to dope the p-type impurities at a very high concentration. Such very high doping lacks in controllability, however. Moreover, the very highly doped region 11 results in increase of the leakage current.